1. Field of the Invention
The present invention generally relates to the manufacture of multi-layer ceramic (MLC) circuit packages and, more particularly, to screening conductive patterns and via filling at small via spacing on wide greensheets.
2. Description of the Prior Art
It has been recognized for some years that miniaturization of electronic circuits produces both manufacturing economies and performance enhancement of the circuits. Small spacing between connected electronic elements results in reduced signal propagation time between circuit elements as well as allowing increased numbers of electronic elements to be placed on a single chip and formed on a single wafer of particular dimensions with a particular series of process steps. Thus the cost of the process can be spread over an increased number of chips of increased performance as well as potentially increased functionality.
Some limits on integration density and circuit miniaturization are imposed by process incompatibility and conflicts for different types of circuit elements. For example, while hybrid BiCMOS devices are known and have been successfully fabricated and used for some purposes, bipolar and CMOS processes are largely incompatible. Similarly, processes for making large capacity dynamic random access memories including potentially millions of small capacitors on or in the substrate are very different from the processes used to form and interconnect transistors in logic arrays. While such conflicts could be approached through masking during chip formation, process complexity is multiplied and manufacturing yield is compromised.
Accordingly, extremely complex electronic circuits have been formed at very small size using multi-layer modular (MLM) packages and multi-layer ceramic (MLC) packages in particular. These packages are formed by forming conductive patterns on insulating lamina with conductive materials filling holes therein to form vias. While polyimide is often used for MLM packages, an uncured ceramic material referred to as a greensheet is used for MLC packages. (A greensheet is of a "rubbery" consistency in its uncured state when screening takes place.) The lamina are then stacked and laminated together to form a robust structure with a complex circuit embedded therein for interconnection of a potentially large plurality of chips which may be formed by independent and potentially incompatible processes. Connections between conductive patterns on respective lamina and to mounting pads for the integrated circuits and connection pins and the like on surfaces of the module are made by the conductive material filling the vias.
It should be understood that the same economic and performance incentives toward higher integration density on a chip, often in combination with somewhat increased chip area, also apply to such modular circuit packages. Therefore, modular circuit packages with increased area and decreased size of vias and spacing between vias have been attempted. At the present state of the art, good manufacturing yields have been achieved with 185 mm wide greensheets having a via spacing of 9 mils, via diameters of 4 mils and greensheet thicknesses of 6 mils. However, acceptable manufacturing yields have not been achieved with a 215 mm wide power plane greensheet having a larger 5.5 mil via diameter for 8 and 11 mil thickness greensheets and a via pitch of 10 mils or less. (The larger diameter vias provided to increase current-carrying capability thus exceed one-half of the via pitch. Note also that the increased lamina thickness either maintains or increases via aspect ratio.) Power planes form a significant portion of MLC packages, particularly where different voltages must be supplied to a plurality of chips in the MLM or MLC package which may be formed by different technologies and require different operating and signal voltages.
On the larger greensheets, the screening paste which is used to fill vias tends to distort the greensheet at the bottom of the vias at pressures sufficient to achieve via filling, producing oversize vias. Oversize vias violate spacing rules and may even become shorted together. The reason for these distortions is not precisely known since it is presently impossible to observe the mechanics and relative motion of the greensheet and paste (between a backing sheet and a mask below a nozzle through which the paste is extruded) during screening but, while not wishing to be held to any particular theory, is probably due, at least in part; to a combination of factors including the greater elastic strain which may occur in a larger greensheet for a given amount of force applied thereto (thus inferring that some aspects of the present processing of 185 mm greensheets have relatively small process tolerances).
Reduction in screening pressure or variation of other screening process parameters to avoid such distortions has uniformly resulted in insufficient via filling for reliable connections to be made between lamina or failure to rectify the problem of oversize and/or shorted vias. As a result, virtually 100% of the wider greensheets included at least one instance of insufficient via fill, spacing violations or shorting between vias and no combination of variation of process parameters and/or known paste formulation was capable of significantly reducing the incidence of such malformations, much less produce an economically acceptable manufacturing yield.
Thixotropic additives are known in formulations of screening pastes for MLC package manufacture. For example, numerous thixotropic additives are disclosed and discussed in U.S. Pat. No. 5,503,777, hereby fully incorporated by reference. Essentially, a suitable paste will contain a conductive powder (and possibly non-conductive fillers) and an organic vehicle including an organic binder. While this paste will exhibit some thixotropy (e.g. the "thixotropic index" is the negative of the slope of the curve of viscosity as a function of shear rate), such a material will maintain a relatively constant viscosity with shearing forces applied thereto and the resulting shear rate.
A thixotropic agent is generally a metal organic ("organometallic") compound which, when added to the above paste at a low concentration, reacts with the organic vehicle. The reaction enhances a property known as thixotropy such that viscosity decreases significantly with increasing shear rate and vice-versa to increase the thixotropic index of the paste. That is, if no force is applied thereto, a thixotropic paste tends to behave as a solid but the material flows increasingly in the manner of a liquid as increased force is applied. Viscosity will often decrease below that of a non-thixotropic paste (e.g. a paste without a thixotropic agent even though it may exhibit some degree of thixotropy) at high shear rates.
Thus, the above-incorporated U.S. patent employs a low concentration (0.5% of the paste while the organic vehicle is generally between or less than 15%-20% of the paste by weight which achieves a three-fold increase in thixotropic index) of thixotropic agent sufficient to prevent flow of paste (referred to therein as "leveling") as the solvent is removed after screening by further processing or absorption by the green sheet while leaving the screening process substantially unaffected. Leveling otherwise produces a dimple in the top of the via due to reduction of paste volume. The above-incorporated U.S. patent also reports a marked reduction in thixotropic index (e.g. reduction in effectiveness of the thixotropic agent to increase the thixotropic index and/or causing a decrease the thixotropic index) as the amount of thixotropic agent is increased (as generally depicted in FIG. 5) from 1.0% to 2.0% for tetrabenzyl orthosilicate and from 2.0% to 4.0% for silicon 2-ethyl-hexanoate (merely doubling the thixotropic index at 0.5% at a 2.0% concentration).
Therefore it has been conventional to maintain the concentration of thixotropic additives very low in order to decrease or avoid increase in effective viscosity during screening so as to properly fill narrow or high aspect ratio vias and properly form fine conductive patterns on the greensheets while presenting a viscosity sufficient to resist internal stresses during further processing after the screening process is completed when no further flow of paste is intended. Nevertheless, no known paste formulation either with or without thixotropic agents has been suitable for achieving even marginally acceptable manufacturing yields on 215 mm or larger power plane greensheets.